In February 2011, a few weeks shy of her 23rd birthday, Rihanna released a single called S&M. The video opens with a sequence showing the singer wearing a dress emblazoned with the words "whore" and "slut" while trapped against a wall by layers of clingfilm. Later, shots of her wearing white latex stockings and suspenders, and brandishing a riding crop are cut with footage where she is prone on the floor, wiggling energetically against the pink rope which binds her hands and feet.

There was quite a furore – this being back in the days before Fifty Shades of Grey had sold 20 million copies. Eleven countries banned the video. Meanwhile, fans jammed arena switchboards worldwide securing tickets to her tour. The following month, she became the youngest black woman to appear on the cover of American Vogue; by the autumn she added British Vogue to her covers, this time dressed as Marilyn Monroe. When Time placed her in its 100 Most Influential list this year, Stella McCartney penned an article in praise of her talent and style. On her Facebook page, 116,541 people have "liked" one album of photos labelled "Hawaii vaca", the aesthetic of which can be summed up as a DIY Sports Illustrated, with more catwalk references. When River Island announced this week that it has signed Rihanna to design a collection that will go on sale on the British high street early next year, the news was acknowledged as a huge coup for the store.

Rihanna's wardrobe is the most talked-about, influential and dissected in pop right now. Beyoncé is mostly accessorised with a baby sling; Lady Gaga has spun off into high-concept theatrics; Katy Perry these days looks like she has wandered off the set of The Muppets. But what Rihanna wears – whether it is red hair, studded cut-off denims, electric blue sunglasses or a new look in nail art – is immediately reproduced on the high street, because it sells.

The Rihanna of 2012 is unrecognisable from the cookie-cutter R&B pop princess who emerged in 2004. Back then she was standard-issue urban-sexy in low-riding jeans, crop tops, tonged hair, hooped earrings; for evening, much the same but with diamante and tighter jeans. But in 2007, the year of her single Umbrella, everything changed. Her hair was dyed black and chopped into an asymmetric bob. In red carpet photos from the Nickelodeon Kids Choice awards that year, you can see the evolution in progress: she is wearing a pretty, multicoloured silk dress, but with a black leather corset belt over the top, and a fashiony front-row scowl instead of a teenybop smile. By 2008, she was rocking chainmail corsets at black tie events, power-shouldered jumpsuits for day and leather hotpants on stage.

"Everytime we write about Rihanna – her nails, clothes, makeup or hair – our traffic goes through the roof," says Carrie Tyler, editor of elleuk.com. "She is a figurehead for gutsy, confident fashion choices. Women are fascinated by her because she takes the risks that her fans would like to take themselves. She's got a beautiful, curvy figure but it's her confidence that appeals as much as her body."

The Rihanna of 2012 is ­unrecognisable from the R&B pop princess who emerged in 2004. Photograph: Jason Laveris/FilmMagic

In 2011, the dominant images of Rihanna in the media were her onstage outfits, which were all leather straps, metal studs, fishnets and fierce heels: female sexuality as visual performance. This year, she has used her Twitter account to overlay this image with a different type of photo, which is equally revealing but very different in tone. These shots have been self-authored, of the singer in a bikini, usually on holiday with girlfriends, accessorised with a bottle of beer and no makeup. This latest Rihanna reinvention (and it won't be the last) is more laidback, apparently closer to reality, than the video dominatrix.

Gemma Cairney, presenter for Radio 1Xtra and fashion stylist, says that "one of the brilliant things about Rihanna is that she gives the impression she doesn't care too much. When she's wearing a see-through bra top on the streets of New York, it feels very celebratory of her body and her sexuality." But Rihanna's provocative wardrobe stirs mixed feelings. "I'm torn on this subject," admits Cairney, "because I love how sexy Rihanna is, and I love that she's got a bottom and thighs with flesh on them and she's out there in shorts, rather than covering up in a flattering pencil skirt. It makes me feel like, maybe you don't have to be a stick insect and you can still feel good about your body. But on the other hand, going out just in your pants – that's fine if you're going to nightclubs with your own private security, but most girls aren't." This year's controversy over Rihanna's apparent rapprochement with Chris Brown, the rapstar boyfriend who assaulted her three years ago, has left many women feeling even more uneasy about whether her hypersexualised image is empowering or depressing.

But Rihanna's look is not just about sex. This is a young woman who loves fashion, and it shows. She is the first mainstream star to showcase the aesthetic of the current festival-going teens: think denim shorts, bright sportswear, kitsch accessories. For her Radio 1 Hackney Weekend appearance she wore an animal-print T-shirt over black shorts, fishnet cycling shorts, with gold jewellery and statement sunglasses; she took the tube in a fire-print oversized T-shirt (by Martin Margiela) and thigh-high leather boots. She regularly wears Gucci, Roberto Cavalli and Giorgio Armani, but adds hipster elements (Boy London) and high street pieces. "Her style is not just about wearing designer dresses," says Cairney. "I like her look best when it has that carnival feel to it: bright and bold, animal prints, rooted in her Caribbean heritage. Then you feel like you could hit Topshop or H&M with her and she'd find something brilliant and be a real laugh."

Malcolm Mackenzie is the editor of We Love Pop, a magazine for 13-15-year-olds. His readers recently voted Rihanna the most stylish modern pop star. "She mixes up pretty and scruffy. It makes her very lifelike to our readers: she's living the dream and keeping it real. And more than any other star, she keeps things fresh by constantly changing her look, which gives the fans something new and keeps her in the spotlight."

Key to Rihanna's appeal is the sense that she is doing what she wants to do and having fun. To what extent her style is a straightforward reflection of this, and to what extent it is manufactured, is not easy to tease apart. When asked about the S&M video in an interview in British Vogue last year, she said: "that's not me. That's a part I play. You know, like it's a piece of art, with all these toys and textures to play with." Her fans, however, ardently want to believe that she is "just being herself" – so that, for the most part, is the message they are given. "There is such an energy about Rihanna. She looks like a young woman who is living her life and doing things her way," says Mackenzie. "Whether or not it is all part of a marketing campaign, the point is that it doesn't come across like that."